The present invention relates to fuel assemblies for nuclear reactors and more particularly to fuel assemblies for reactors cooled with light water.
Even more specifically the present invention relates to improvements to the guide tubes of such fuel assemblies and brief details will be given below of the construction thereof.
Reference can be made to French Patent Nos. 1 536 527 and 70 18 102 in the name of the Westinghouse Electric Corporation for details of fuel assemblies for light water reactors of a known type. In components of the type described in these Patents the nuclear fuel of the reactor is distributed over a plurality of fuel rods located in the reactor core, the contiguous assemblies having a generally square cross-section. Each fuel assembly extends over a length of several meters and has a rigid mechanical frame, being held between the upper and lower core plates. This frame comprises an upper and a lower end part provided with orifices permitting the passage of the cooling water which circulates vertically from bottom to top and a certain number of guide tubes which, through their fixture to the upper and lower end parts, ensure the strength and stability of the assembly. Along each assembly there is a certain number of supporting grids fixed to the guide tubes and defining cells with a square cross-section permitting the passage and the transverse support of the actual fuel rods which are, so to speak, suspended by friction in the grids without contact with the upper and lower end parts.
As is known, in addition to their mechanical supporting function with respect to the assembly, the guides tubes serve to house the absorbent rods which can be used at any time for checking the chain reaction and which are inserted as rapidly as possible into the assembly across the upper part which is perforated for this purpose when, for any random reason, it is desired to bring about a rapid stoppage of reactor operation.
Conventionally the end parts of the fuel assembly are made from stainless steel or Inconel, i.e. materials having a relatively high neutron absorption, whilst the guide tubes are made from zircaloy, which has much more favourable neutron properties.
In the known fuel assemblies referred to hereinbefore it is also necessary to provide for the guide tubes braking devices which, during the rapid shutdown of the reactor, make it possible to significantly decelerate a speed at which the control rods drop at the end of travel thereof in their different guide tubes. In the hitherto known constructions this braking action is obtained by a constriction in the lower portion of the guide tube, e.g. at a certain height above the lowest grid and orifices are positioned above this constriction area to permit the removal from the guide tube of the water displaced during the fall of the control rod.
Moreover, in said same hitherto known solutions, the guide tube is fixed to the upper end part of the assembly by means of an intermediate sleeve made from a material compatible with that of the end part, because it is not possible to directly weld the zircaloy guide tube to the steel or Inconel part. As a result it is necessary to use a steel sleeve fixed to the outer surface of the upper portion of the guide tube and held by means of a certain number of mechanical deformations ensuring the fixture of the sleeve to the guide tube. The sleeve is itself welded or brazed on the one hand to the upper end part and on the other hand to the upper grid, thus ensuring the mechanical connection of the end part to the overall structure.
However, these arrangements lead to a certain number of serious disadvantages, which will be enumerated hereinafter.
Firstly the guide tubes must be manufactured in two parts of different diameters connected by a generally conically constricted area, which is difficult and requires particularly precise tooling and production controls. Moreover, the braking obtained in this way at the end of the drop of the control rods is not progressive and occurs relatively suddenly on clearing the constriction, which, in certain cases, can be highly disadvantageous for the rod. In addition, the use of a connection by welding or brazing between the upper grid and the upper end part by means of a steel sleeve imposes, for reasons of metallurgical compatibility, a steel or Inconel grid, which is highy prejudicial to the neutron balance of the reactor.